TrulyMail Portable is designed to be installed on removable media (USB drive, flash card, etc.) and is only usable by one person (unlike TrulyMail Standard). It is portable because you can easily move it from one computer to another.
Note: Your messages are only encrypted in transit - they are not encrypted when stored on your computer or removable media. If you want to store your local information (data from TrulyMail and other applications) in a secure way, we recommend you consider a standalone data encryption application like TrueCrypt or BestCrypt (these tools allow you to encrypt part of your hard drive so nobody but you can access your files).